1. Field of the Invention
The present invention is related to photonic devices and, in particular to hybrid integrated optoelectronic circuits and optical packaging
2. Background Information
Hybrid integrated optoelectronic circuits are commonly fabricated using discrete active and passive photonic devices and electronic devices. Active photonic devices commonly include edge-emitting lasers such as Fabry-Perot (FP) and distributed feedback lasers (DFB), vertical cavity surface-emitting lasers (VCSEL), electro-absorption modulated lasers (EML), photodetectors, semiconductor optical amplifiers, electro-absorption modulators, and other discrete photonic devices. Active optoelectronic (photonic) devices could be combined on the same chip with electronic circuits (devices), for example laser diode and laser driver circuit, and/or photodetector and transimpedance amplifier could be integrated on one chip. At the same time discrete photonic and electronic devices could be bonded to a common substrate, which provides electrical and optical interconnects for the devices. Passive photonic components include guided wave optical elements and optical fibers. Passive photonic components include 1xc3x97N (N=2,4,8,16) optical splitters, combiners, wavelength division multiplexed (WDM) or coarse-wavelength division multiplexers or demultiplexers (CWDM/wideband WDM), variable optical attenuators. To fabricate hybrid optoelectronic circuits, passive and active photonic devices typically are formed in a chip substrate using chemical processes and/or are bonded to the substrate using solders or epoxies.
In the prior art, light is coupled from separately packaged photonic devices into optical fibers and then coupled from the optical fibers to separately packaged waveguide devices. To ensure optimal coupling of light, each photonic device is aligned with its associated optical fiber and each optical fiber is aligned with its corresponding input waveguide of guided wave device.
One alignment technique involves a self-alignment in which the surface tension force of melted solder pulls each optoelectronic component being bonded to common substrate (or optical bench) into alignment position and when cooled the solder provides mechanical and electrical connection of the optoelectronic component (e.g., laser) die to a bonding pad on the common substrate. In some instances, micro-machined mechanical stops further refine the alignment in that they restrict the movement of the discrete components.
Another technique involves active alignment, which involves, for example, powering the laser so that it emits light, coupling the light into a waveguide, and monitoring optical power at the output of the waveguide to determine whether light was coupled efficiently. This is a widely used technique for packaging lasers/photodetectors with optical fiber for telecom applications. The optoelectronic device (e.g., laser/photodetector) is fixed onto a ceramic substrate. Micro-optic components such as isolators and lens are then placed in front of the laser. The fiber is then welded into place after optimizing its position to achieve optimum (i.e.,  greater than 50%) coupling of light between the optical fiber and the optoelectronic device by active alignment. Typically active alignment is a time-consuming, largely manual process that requires expensive equipment, resulting in the packaging dominating the cost of the optoelectronic module.
With the gradual migration of optical links from long-haul transport to the enterprise and eventually the desktop, there is a growing need for highly functional optical and optoelectronic components that occupy a small form-factor, and that are inexpensive. New optical packaging approaches need to be developed in order to address this need.
For instance, assembly of small form-factor multiple component hybrid optoelectronic devices requires flip-chip bonding of several active optoelectronic components with fine pitch and high after-bonding alignment accuracy. Currently there are volume manufacturing ready Flip-Chip Bonders that can place an optoelectronic die onto the two-dimensional surface of the substrate with placement accuracy of xc2x11 xcexcm available, with much shorter cycle times compared to active alignment. In order for edge-emitter devices to couple to each other and onto optical fiber, similar accuracy is required for die placement in the vertical (xe2x80x9czxe2x80x9d) direction. To achieve alignment accuracy in the xe2x80x9czxe2x80x9d direction, special mechanical pedestals are micro-machined on the substrate.
Such techniques offer the possibility of high accuracy active optoelectronic component die bonding without using active alignment or relying on self-alignment effect. However heating of the substrate during bonding of subsequent components (e.g., laser dies) can affect the alignment of the previously bonded components. That is, the cooled (solidified) solder bond on the previously bonded component die melts when the substrate is heated to bond the next component die and the previously bonded die moves, thereby affecting its optical coupling to a passive waveguide on the substrate or coupling to/from optical fiber.